Refrigeration system with means to prevent compressor surge



9, 1966 w. "r. osBoRNE 3,264,843

REFRIGERATION SYSTEM WITH MEANS TO PREVENT COMPRESSOR SURGE Filed June23, 1964 2 Sheets-Sheet 1 INVENTOR. WILLIAM T. OSBORNE.

"WWWW' ATTORNEY.

Aug. 9, 1966 W. QSBORNE 3,264,341

REFRIGERATION SYSTEM WITH MEANS TO PREVENT COMPRESSOR SURGE Filed June23, 1964 2 Sheets-Sheet 2 FULLY OPEN MODULATING FULLY CLOSED ENTERINGconosusms wATER TEMPERATURE SURGE CHARACTERISTICT DEGREES F. STEAMCONDENSATE TEMPERATURE DEGREES F. 10-

0 20 40 so 80 I00 I20 I. NOMINAL COOLING CAPACITY FIG. 2

INVENTOR. WILLIAM T. OSBORNE.

ATTORNEY.

United States Patent 3,264,841 REFRIGERATION SYSTEM WITH MEANS T1)PREVENT COMPRESSOR SURGE William T. Osborne, Syracuse, N.Y., assignor toCarrier Corporation, Syracuse, N.Y., a corporation of Delaware FiledJune 23, 1964, Ser. No. 377,316 8 Claims. (Cl. 62-196) This inventionrelates to means for effectively preventing compressor surge in arefrigeration system and, more particularly, to a hot gas bypass controlfor a variable speed centrifugal refrigerant compressor.

Various expedients are known in the refrigeration field for preventingcompressor surge in a refrigeration system. For example, a hot gasbypass having a normally closed valve may be provided between the highand low pressure sides of the system.

In a refrigeration system disclosed in a copending patent application ofLouis H, Leonard for a heating and cooling system, application No. 377,258, and filed on the same date as the present application, neither thetemperature nor flow of condensing water need be controlled. However, asthe temperature of condensing water changes, the surge pointcharacteristic of the refrigerant compressor changes. For example,should the condensing water temperature rise, the refrigerant pressureon the high side of the system will rise, and may eventually cause thecompressor to become unstable, or to surge, with little or no decreasein compressor speed or refrigerant flow. Should the condensing Watertemperature drop, the speed or flow may be decreased a much greateramount before surge occurs. The cooling capacity of the system describedin the abovementioned copending application is regulated by regulatingthe discharge pressure of a steam turbine which drives the compressor tovary the refrigerant output of the compressor. Steam from the turbinedischarge passes into a steam condenser, and the turbine discharge steampressure is regulated by controlled blanketing of a condensing portionof the steam condenser with a noncondensible vapor, preferablyrefrigerant vapor. For example, when it is desired to reduce the coolingcapacity of the system, the output of the turbine and compressor isreduced by increasing the blanketing of the steam condensing portion,thus reducing its condensing capacity and raising the turbine dischargepressure to reduce the turbine and compressor output.

The present invention is directed to etfectively prevent compressorsurge in such a system responsive to a steam condensate temperaturerange.

It is a primary object of this invention to provide a new and improvedrefrigeration system. A related object is provision in such a system ofa new and improved surge prevention control for a refrigerant compressorand, more particularly, for a compressor in a refrigeration system asdescribed in the aforementioned copending patent application. A relatedobject is provision of a surge prevention control which is responsive tosteam condensate temperature.

A more specific object is to provide a new and improved surge preventioncontrol for a refrigerant compressor in a refrigeration system wherein arefrigerant side of the system includes a hot gas bypass operable foreffectively preventing compressor surge, and a power side of the systemincludes a steam turbine for driving the compressor and dischargingsteam into a steam condenser, the output of the turbine and compressorbeing regulated inversely of the cooling capacity of the system and thesteam condensate from the turbine being at a substantially predeterminedtemperature as the compressor approaches surge condition, and a sensoroperable responsive to the steam condensate approaching thepredetermined temperature to operate the hot gas bypass for effectivelypre venting surge of the compressor.

Additional objects and advantages of the invention will be apparent fromthe following description and drawings in which:

FIGURE 1 is a flow diagram of a refrigeration system incorporating apreferred embodiment of a compressor surge prevention control;

FIGURE 2 is a graph illustrating compressor surge and surge controlcharacteristics in the system.

The illustrated refrigeration system is preferably air tight and may beconsidered as having a power side including a circuit for thecirculation of power fluid, and a refrigerant side including a circuitfor the flow of refrigerant under the influence of drive means driven bythe power fluid, with the operation of the system regulated by a controlsystem.

The invention will be described with reference to a preferred powerfluid which is water, and a preferred refrigerant which isoctafluorocyclobutane, commonly referred to as C318 and having achemical formula C 1 These fluids are particularly preferred because oftheir relative immiscibility and because they are inherently highlystable and do not tend to decompose or chemically react with each otheror other materials in the system, or cause or promote corrosion andundesirable byproducts. Also, this refrigerant is a relativelynoncondensible vapor at the temperatures and pressures at which thepower fluid (water) condenses as Well as at the usual ambientatmospheric conditions of temperature and pressure. However, other powerfluids and refrigerants having the desired chemical and physicalproperties may be utilized within the scope of this invention.

With reference to FIGURE 1, the power side of the refrigeration systemincludes a suitable steam generator 11 Which supplies steam through asteam supply line 12 to steam powered drive means here in the form of aturbine section 13 of a turbocompressor 13. The turbine 13 dischargessaturated steam into a discharge steam line 15 opening into a steamcondenser 16. The steam is condensed by a first condensing portion ortube bundle 17, and a second condensing portion or tube bundle 18 whichprovides heat to a load to be heated. A heating water pump 18'recirculates heated water from the second condensing portion to theload. Steam condensate in the condenser 16 passes through a port 19 andinto a steam condensate chamber 20. From the chamber 20 the steamcondensate is returned through a steam condensate line 21, by means of asteam condensate pump 22 in the line, to the steam generator 11 forrecirculation through the power side of the system. In the illustratedembodiment, the steam is supplied to the turbine at a substantiallyconstant pressure, for example 15 p.s.i.g., as by means of a suitableconstant pressure steam regulating valve 23 in the steam supply line 12.

The turbocompressor 13' is preferably provided with water lubricatedbearings. The steam condensate pump 22 passes steam condensate through alubricant line 24 and cooling means 24' to the turbocompressor, fromwhich the lubricating water and any leakage of refrigerant or steamwithin the turbocompressor is returned to the steam condensate chamber20 through a return line 25.

The refrigerant side of the system includes a refrigerant compressorsection 31 of the turbocompressor 13', and preferably a centrifugalcompressor for discharging relatively high pressure refrigerant vaporfrom the compressor outlet 32 into a refrigerant line 33 opening into arefrigerant condenser 34. The refrigerant condenser 34 has a condensingportion or tube bundle 35 for circulating condensing water to condensethe refrigerant vapor. Refrigerant condensate passes from therefrigerant condenser through a refrigerant condensate line 36 to arefrigerant subcooler 37 and then through a line 39 to suitablerefrigerant flow control means 40, here in the form of a float valveassembly. In keeping with normal practice, an equalizer line 40 may beprovided between the refrigerant condenser 34 and the chamber of theflow control means 30. The refrigerant then passes through a coolersupply line 41 and into a cooler 42, and more particularly, into arefrigerant pan 43. The pan 43 contains a flooded chilled water tubebundle 44 communicating with a chilled water line 45 having a chilledwater pump 46 for circulating chilled water to a load to be cooled.Boiling refrigerant in the pan vaporizes into a refrigerant chamber 47of the cooler and is withdrawn through a suction line 49 to an inletconnection 49 of the compressor 31. The portion of the refrigerant sideof the system between the compressor outlet 32 and the float valve 40defines a high pressure side, and the portion between the float valve 40and the compressor inlet 49' defines a low pressure side.

The refrigerant side of the system further includes a hot gas bypassoperable for effectively preventing compressor surge and including a hotgas bypass line 55 cnmeeting the cooler supply line 41 and an upperportion of the refrigerant condenser 34 for passing refrigerant vaporaround the float valve 40 when a normally closed modulating refrigerantvalve 56 in the bypass line 55 is open.

Cooling capacity control of the system is regulated by regulating therefrigerant output of the compressor 31 which is connected with theturbine 13 by a shaft 60. In order to regulate the power output of theturbine 13, the pressure within the steam condenser 16 is regulated.Refrigerant, which is a noncondensible vapor within the steam condenser16 is passed through a refrigerant line 62 from the refrigerant chamber47 of the cooler 42 and through an inlet port 63 in the steam condenserto blanket the first condensing bundle 17 of the steam condenser withrefrigerant vapor and reduce the condensing capacity, thereby raisingthe condenser pressure to reduce the turbine output, as is more fullydescribed in the previously mentioned Leonard application.

A baffie 64 between the tube bundles 17 and 18 extends longitudinallythrough the steam condenser in sealed engagement with a steam condensershell 64' except at a limited area of communication 65 at therefrigerant port 63 and at an end of the bundles opposite the outletport 19 and the opening of the steam discharge line into the upperportion of the steam condenser, so that the second bundle 18 is free ofrefrigerant vapor to provide maximum possible heat output, while thefirst bundle 17 is blanketed by refrigerant vapor. Steam condensatepasses through the outlet port 19 and into the steam condensate chamberfrom which the condensate is returned to the steam generator 11.

Refrigerant vapor is withdrawn from the steam condenser 16 by means of apurge line 70 opening into the steam condensate chamber 20. An oppositeend of the purge line 70 opens into the throat of a jet pump 71 in alower water sump portion 72 below the pan 43 within the cooler 42. Waterentering the cooler 42 is separated from refrigerant and collects in thesump 72, as is more fully described in the previously mentioned Leonardapplication and in my copending United States patent application for acooler, application No. 377,317, and filed on the same date as thepresent application. Water is recirculated through the sump by asuitable water supply pump 73 which provides impeller water for the jetpump 71. In the preferred embodiment, the water supply pump 73 operatesat constant speed so that the purge line 70 withdraws refrigerant vaporfrom the steam condenser at a substantially constant rate. A modulatingrefrigerant valve 75 in the refrigerant line 62 is preferably controlledby a suitable temperature sensor 76 on the chilled water line 45 so thatthe quantity of refrigerant entering the steam condenser, and thereforethe turbine and compressor outputs, are controlled responsive to chilledwater temperature.

Make-up water for the power side may be provided by a make-up water line77 from the water supply pump 73 to the steam condensate chamber 20. Afloat sensor 78 in the chamber 20 operates a valve 79 in the line 77 toadd make-up water as needed.

A suitable condensing medium, such as tower water, is circulated by atower water pump 80 from a supply line 81 connected with the tower (notshown) and through the refrigerant subcooler 37 and an intermediate line82 to the refrigerant condenser tube bundle 35 and then the steamcondenser first tube bundle 17, and back to the tower through a returnline 83. Thus, the tower water is serially circulated through therefrigerant condenser and then the steam condenser.

Neither tower water temperature nor flow rate need be regulated duringoperation of the system. However, as the tower water temperature rises,it can effect less condensing and compressor surge may occur. Therefrigerant pressure in the refrigerant condenser 34 rises, thusincreasing the compressor discharge pressure. Also, the steam condenserpressure increases thus increasing the turbine discharge pressure andthe temperature of the steam entering the steam condenser to increasethe steam condensate temperature.

Liquid subcooling is a characteristic of condensers containingnoncondensibles and is defined as the difference in temperature betweenthe liquid leaving the condenser and the actual condensing temperatureinside the condenser. Subcooling occurs by virtue of the inability ofthe saturated vapors to reach the colder tubes which are blanketed bynoncondensibles and the tremendous capability of the blanketed, coldertubes to cool the droplets of liquid condensate as they drop, bygravity, from higher non-blanketed tubes, or as the droplets are carriedinto the blanketed tubes by the motion of the vapors and gases in thecondenser. Generally, the amount of subcooling which occurs isproportional to the quantity of noncondensibles present and effectivelyblanketing the condensing tubes, and to the difference between theactual condensing temperature and the temperature of the water passingthrough the condenser tubes. The steam condensate temperature is afunction of the amount of subcooling and the actual condensingtemperature. However, as the refrigerant condenser pressure increases,the compressor 31 must pass a greater quantity of refrigerant throughthe system at a greater pressure differential in order to maintain thesame cooling capacity. Thus, the output of the compressor 31 must beincreased, resulting in the refrigerant condenser pressure increasingand requiring a further increase in compressor output. In order toincrease the compressor output, the turbine discharge pressure must bereduced by reducing the blanketing of the first condensing bundle 17,resulting in a reduction of the partial pressure of the noncondensiblerefrigerant vapor and a reduction of condensate subcooling. The endresult is that the temperature of the steam condensate leaving the steamcondenser 16 rises appreciably as the compressor approaches its surgepoint.

Another condition which may result in compressor surge is low coolingcapacity at which time a relatively small quantity of refrigerant vaporis passed by the compressor '31 so that the compressor speed isnecessarily slow. Thus, the turbine must operate at slow speednecessitating a high turbine discharge pressure resulting from verygreat blanketing of the steam condenser first condensing bundle 17, thusproducing a high steam condenser pressure w-hich results in high turbinesteam exhaust temperature and a relatively high steam condensatetemperature.

Therefore, in order to retard the tendency of the compressor 31 to surgeunder such conditions, the temperature of steam condensate leaving thesteam condenser 16 may be sensed for opening and regulating the amountof opening of the hot gas bypass valve 56. The temperature of steamcondensate leaving the steam condenser is sensed by a temperature sensor85 in the steam condensate chamber 20. The sensor 85 is preferably aselfcontained portion of the hot gas bypass valve 56.

With reference to FIGURE 2, the surge characteristic of the compressor31 and a family of steam condensate temperature curves is plottedagainst entering tower or condensing water temperature and percentage ofnominal cooling capacity. If the turbocompressor is operated withoutsuitable surge prevention means, the compressor will surge at anycombined condition of tower water temperature and percentage coolingcapacity to the left of the surge characteristic curve. The family ofsteam condensate temperature curves closely parallel the surgecharacteristic curve, and the sensor 85 in the steam condensate chamber20 is adjusted to open the hot gas bypass valve 56 when the steamcondensate temperature in the chamber reaches 122 F, for example. At asteam condensate temperature of 126 F., for example, the hot gas bypassvalve 56 is fully open and modulates between the closed and full openpositions in proportion to the temperature of the steam condensatebetween 122 F. and 126 F. Because the condensate temperature and surgecharacteristic curves closely parallel each other,.

greater operating efficiency of the system throughout a greater range ofoperating conditions may he obtained by means of the subject surgeprevention control.

The following chart indicates various operating conditions throughoutthe system:

Nominal Cooling Capacity 100% 50% Entering Condensing Water,

Lv. Chilled Water, F 44 43 42 41. 40. 5 40 Cooler:

F 36 35 34 33 32. 5 32 p.s.i.a 19. 7 19. 2 18. 7 18. 2 18 17. 7 SteamCondenser:

105 85 95 75 86 66 p.s.i.a 70 51 60 43 52 36 Ref. Leaving Subcooler, F95 75 90 70 86 66 In summary, during operation of the refrigerationsystem with relatively high tower water temperatures, a reduction ofrequired cooling capacity is sensed by the control valve sensor 76,which regulates the rate of bleeding noncondensibles into the steamcondenser to raise the turbine discharge pressure and reduce the turbinepower output. As the cooling load continues to decrease, and the steamcondenser pressure is correspondingly increased, the compressorapproaches a surge condition. At the same time, the steam condensatetemperature, which is a function of steam flow, unblanketed steamcondensing surface, and tower water temperature in the steam condenser,rises to a relatively high determinable level. During conditions ofdecreasing capacity with relatively low tower water temperature, thesteam condenser pressure is increased more for each reduction ofcapacity since less turbine power is required at each capacitycondition. However, since the steam condensate temperature is influencedby the temperature of the tower water in the steam condensing bundle, itincreases less rapidly with capacity reduction at low tower watertemperatures. When the compressor reaches surge condition with lowerentering tower water temperature, the steam condenser pressure ishigher, the cooling capacity is less, and the steam condensatetemperature is the same as with high tower water temperatures.

It should be noted that in the foregoing system, the lower the towerwater temperature the lower may be the partial 'load operation of thecompressor without entering the surge range. With relatively cold towerwater, much lower cooling capacity of the system is necessary to producea sufficiently high steam condensate temperature for operating the hotgas bypass valve. Should the temperature of the tower water increase,the condensing capacity of the steam condenser will be reduced so thatless blanketing of the steam condenser is required to maintain a desiredturbine discharge pressure for a particular cooling capacity of thesystem, and while the temperature of the discharged steam remains thesame, the higher temperature of the steam condensing portion tends toproduce a higher steam condensate temperature. Thus, as compressor surgecondition is reached the steam condensate approaches a relatively highdeterminable temperature.

While a preferred embodiment of the invention has been described andillustrated it will be understood that the invention is not limitedthereto since it may be otherwise embodied within the scope of thefollowing claims.

Iclaim:

1. In a refrigeration system, the combination comprising, a refrigerantcompressor, first means operable for effectively preventing compressorsurge, a steam condenser, steam powered drive means for driving saidcompressor and discharging steam into said steam condenser, second meansfor regulating the output of said drive means and thereby the output ofsaid compressor in proportion to the cooling capacity of the system andas said output is reduced, for raising the steam condensate to asubstantially predetermined temperature as the compressor approachessurge condition, and control means responsive to said predeterminedcondensate temperature for operating said first means and efiiectivelypreventing surge of said compressor.

2. The system of claim 1 wherein the output of said compressor isproportional to the output of said drive means, the drive means outputbeing variable inversely of its discharge pressure, and said secondmeans including, means for passing a nonoondensible vapor into saidsteam condenser to regulate the steam condenser pressure and steamcondensate temperature, and means for regulating the quantity of saidnonoondensible vapor in said steam condenser inversely of the coolingcapacity of the system.

3. The system of claim 2, and a refrigerant condenser on a high side ofthe system and a cooler on \a low side of the system, the refrigerantand steam condensers having condensing portions for circulation of acondensing medium, whereby at a relatively high entering condensingmedium temperature the refrigerant condenser pressure is relatively hightending to cause compressor surge and the steam condensate temperatureis relatively high as the compressor approaches surge condition.

4. The system of claim 3 wherein said condensing portions are in series,whereby said condensing medium is serially circulated through saidcondensing portions.

5. The system of claim 4, and means for circulating said condensingmedium through the refrigerant condensing portion first.

6. The system of claim 5, and said first means comprising hot gas bypassmeans between said high side and said low side.

7. The system of claim 3 wherein said control means operates said hotgas bypass means for passing refrigerant at a flow rate in proportion tothe steam condensate temperature above said predetermined temperature.

8. The system of claim 7 wherein said compressor is a centrifugalcompressor.

References Cited by the Examiner UNITED STATES PATENTS 2,183,821 12/1939Nelson 62500 X 2,983,111 5/1961 Miner 62-228 X MEYER PERLIN, PrimaryExaminer.

1. IN A REFRIGERATION SYSTEM, THE COMBINATION COMPRISING, A REFRIGERANTCOMPRESSOR, FIRST MEANS OPERABLE FOR EFFECTIVELY PREVENTING COMPRESSORSURGE, A STEAM CONDENSER, STREAM DRIVE MEANS FOR DRIVING SAID COMPRESSORAND DISCHARGING STEAM INTO SAID STEAM CONDENSER, SECOND MEANS FORREGULATING THE OUTPUT OF SAID DRIVE MEANS AND THEREBY THE OUTPUT OF SAIDCOMPRESSOR IN PROPORTION TO THE COOLING CAPACITY OF THE SYSTEM AND ASSAID OUTPUT IS REDUCED, FOR RAISING THE STEAM CONDENSATE TO ASUBSTANTIALLY PREDETERMINED TEMPERATURE AS THE COMPRESSOR APPROACHESSURGE CONDITION, AND CONTROL MEANS RESPONSIVE TO SAID PREDETERMINEDCONDENSATE TEMPERATURE FOR OPERATING SAID FIRST MEANS AND EFFECTIVEPREVENTING SURGE OF SAID COMPRESSOR.